This invention relates to apparatus for use in a borehole, and more particularly, relates to nuclear magnetism logging apparatus for use in an earthen borehole.
The most accurate open borehole logging device for measuring residual oil saturation and permeability in earth formations is the Nuclear Magnetism Log (NML). This logging tool uses the magnetic field from a solenoidal coil to polarize the Protons in any fluids contained within the earth formation adjacent the tool. The solenoidal coil is then turned off and any mobile protons influenced by the magnetic field of the solenoidal coil precess at their Larmor frequency about the earth's magnetic field. This precession may be measured as a damped sinusoidal voltage induced in a separate detection coil in the logging tool. The induced voltage decays rapidly, tyPically of the order of 20-50 milliseconds, because of extremely short spin-spin or transverserelaxation times, T.sub.2 *, of these fluids.
The initial amplitude of the nuclear magnetism signal is proportional to the "free-fluid" or producible fluids in the formation adjacent the tool. If Mn-EDTA is added to the drilling fluid and allowed to invade the formation, then the only residual nuclear magnetism signal will be from any oil Phase in the formation adjacent the tool. Thus, a NML log-inject-log procedure results in highly accurate measurements of residual oil saturation in the formation.
The spin-lattice or longitudinal relaxation time T.sub.1 of any fluid in an earth formation is related to the pore size(s) (and their distribution) containing the fluid; the pore sizes and distribution may then be related to the capillary pressure curve which may then be related to the permeability of the formation. (See for example, Loren, J. D. and Robinson, J. D., "Relations Between Pore Size, Fluid and Matrix Properties, and NML Measurements", Society of Petroleum Engineers Journal, September 1970, pp. 268-278 and Loren, J. D., "Permeability Estimates From NML Measurements", Journal of Petroleum Technology, August 1972, pp. 923-928.) The NML attempts to measure T.sub.1 by repeated polarization cycles with successively longer polarization times. Since T.sub.1 is very much longer than the time T.sub.2 * of the damped sinusoidal decay, T.sub.1 cannot be determined directly from the damped sinusoidal decay curve with only one measurement.
Although a highly useful logging tool, one of the main disadvantages of the NML is its poor signal-to-noise ratio, which limits its accuracy to about 1 unit of porosity during continuous logging operation. Although this is adequate for measuring a free-fluid index, residual oil saturations are typically 1/3 or less of the total porosity, so that stationary NML operation is required in order to measure the residual of saturation to sufficient accuracy for enhanced oil recovery requirements. Typically, about 15 minutes of data is collected at a borehole location and then averaged to obtain residual oil saturation to better than 1 percent of saturation at that location. However, maintaining the tool stationary for this length of time slows data acquisition for the entire formation and increases the risk of sticking the tool in the borehole.
Another disadvantage of the NML is the shallow depth of investigation into the formation. Again, due to poor signal-to-noise ratio, only signals from the initial few inches of the borehole wall can be detected.
Still another disadvantage is that repeated polarization cycles are required to obtain a discrete number of measurements of the T.sub.1 decay curve. This requires additional time, and since only a few measurements are obtained, the shape of the T.sub.1 decay curve so obtained is not very precise. This T.sub.1 decay curve then contains imprecise information on pore sizes and their distribution (and accordingly any capillary pressure or permeability determined therefrom is imprecise) in the formation, which is a severe disadvantage of the present NML tool.
The use of a Superconducting Quantum Interference Device (SQUID) as a detector in a modified nuclear magnetism tool has been proposed, with the modification being the use of two opposed superconducting magnets instead of the conventional polarizing solenoid to provide an increased depth of investigation into the borehole wall (J. A. Jackson, "New NMR Well Logging/Fracture Mapping Technique With Possible Application of SQUID NMR Detector" SQUID Applications to Geophysics, Proceedings of June 2-4, 1980 Workshop, los Alamos, N. Mex., pp 161-164, published by SEG, 1981). However, the use of a SQUID as a detector in a downhole device has not occurred because of the cryogenic and safety considerations of using liquid helium in the borehole environment. More specifically, liquid helium expands over 600 times on vaporizing and cannot be safely vented into the borehole. This problem, as well as that of providing an adequate amount of operating time downhole, in the high-temPerature environment oF a borehole, has so far prevented the use of a SQUID as a detector in a nuclear magnetism tool.
These and other limitations and disadvantages of the prior art are overcome by the present invention, however, and apparatus are provided for nuclear magnetism logging with a detector capable of detecting sinusoidal and slowly varying magnetic fields in an earthen borehole.